Velocity question

[tcarr]

I am trying to work up some loads for a 308 and a 300 win. The first loads I shot last weekend shot well, but the second load for the 300 win shot about 50 fps slower. I thought maybe I didn't label my loads correctly, and didn't think too much about it.

Shot today with different loads in both rifle again. The 1st 300 win load was 1 grain higher than the first I shot on Saturday, but was the same velocity, and the second load, 1 grain higher still, shot slower by 25-30 fps. I also went up on the 308 and it shot 200 fps slower than the load that had .8 less grains of powder.

The temp today was 20 degrees cooler, but that should have only dropped it 20 or fps. Anyone ever have this issue, and if so what did you ocme up with?

[somerled]

The temperature will make a huge difference in velocities. Some powders are more likely to be erratic with temperature deviations. Also, ammunition that has set in a cold truck will perform differently than the very same stuff you've just brought out of a warm building.

Reloading and getting good results is all about controlling variables. Different lots of the same powder will burn differently. Some powders give erratic results when the cases aren't as full, for example. How cases are sized can impact their capacity and therefore chamber pressures. These are just a few of the variables that can impact velocities from one range session to the next.

[Unclenick]

While you're at it, don't forget to control the chronograph. A lot of them can read those kinds of velocity number differences if the light conditions change too much.

To check whether temperature is the problem, just keep the test loads in your shirt pocket until you use them. If the gun has already been warmed up with a couple of rounds, the heat lost from the powder during transfer from your pocket to the chamber won't amount to much.

Nick